This invention is in the field of electric power generation, and more particularly, relates to induction generator systems.
Virtually, all electric power generators in current use are synchronous machines. Such generators are typically connected together to form an electric power grid. In other cases, synchronous generators are operated as autonomous electric power generators. While such synchronous machines do effectively perform in the required electrical power generating applications, those machines are relatively high cost compared with other known generators such as induction machines adapted for operation in a power generation mode.
In the application of induction machines to power generation, an induction machine is generally driven by a prime mover or torque generator, such as an internal combustion engine. The torque generator has an output shaft which is coupled to drive the input shaft of the induction generator. Generally, in the prior art, the electrical frequency of the power produced on the output lines of the induction generator is compared with a reference, for example, 50 Hertz, and the frequency differential is used to control the torque produced by the torque generator in a closed loop fashion. A common form of this control system uses a controlled throttle between an internal combustion engine and its fuel supply as the input variable.
Since this control configuration feeds back around the cascade combination of the torque generator and the induction machine, the closed loop bandwidth must be lower than that possible for the torque generator taken alone. Although this relatively low bandwidth control system can keep the average value of the induction generator output frequency correct, it will not have high immunity to local disturbances, such as may be due to local load switching. For example, when a large electrical load drops off, the torque generator's load can fall towards zero in a fraction of a cycle of the electrical output. During the one or two output electrical cycles before this frequency control loop can respond to the load change, the excess torque supplied by the torque generator will result in too large an input shaft frequency to the induction machine. The combination of the induction machine's slip frequency going to near zero, and the over frequency on the induction machine's shaft input, can result in a substantial output frequency overshoot. This undesirable output frequency overshoot can many times turn into a relatively long time over-frequency condition since, although prime movers can supply torque, most cannot actively slow down except through minimal frictional losses. This relatively long time overfrequency condition can also create problems for the voltage control loop.
It is an object of the present invention to provide an improved system and method for controlling an induction generator system.